Have some problems with getting 1 screw out from my surfboard. It has no grip anymore, its just rounded hole ... so no screwdriver in my tool box can get a grip .... any tips on how to pull that screw out?

If the screw is not countersunk, you can simply saw a new notch in the head.
Very easy !

Assume it is for footstraps and not fins, as noone (except for Slingshot) uses screws for surfboard fins.

If you got a footstrap where you can NOT saw a notch for some reason, then you have another alternative:

If you have access to one at a workshop, you can use or borrow a "screw extractor".

This is a narrow diameter screw tap with left handed thread, so you drill a hole in your screw, and can turn the screw out by means of this left handed screw tap, by simply cutting a new thread counterclockwise in your drilled hole.

Otherwise - if you have some kind of "Dremel" tools or similar - you can try to make a small slot/notch in the head, without wrecking the board (not easy though...) - but might not be possible as the stainless screw is extremely hard.

Finally, if it is a mini-tuttle box fin, you can drill the head "away" and ditch the fin and use a new one instead.

A small drill bit and a screw extractor is a fairly cheap method that can prevent damage to the surrounding equipment. I've used 'em without the fancy holder before - just a pair of locking pliars... Use the size of drill bit specified on the package. The key for me was to tap it in with a hammer before turning.

Aaah, sorry, I did not get the "topic headline" which said it was the fin screws loud and clear - my mistake...

So it is the small "Allen key" fin screws.

Still, a small screw extractor might do it.

To prevent the hex from damage, one has to be careful, as there are two almost identical screws, but the Allen keys dont match

One is the most used Future and FCS grub screws which are typical US/Imperial, whereas Naish uses a Metric size for their FCS/DFS fins.

So one is 2.38mm (3/32"), the other is 2.5mm.

And they dont fit into each other - and such a small difference that you can almost not measure it

We have often bothered A LOT with this, when on wave trips together, and "borrowing" Allen keys from each other

I've also seen some "cheap" fin keys, that are not 3/32 nor 2.5 and looks like something not belonging to any standard
They also tend to be damaged really easy - too soft and not hardened material, so they get messed up fast